OBJECTIVES: I. Determine the maximum tolerated dose of cladribine in combination with radiotherapy in patients with high grade glioma. II. Assess this regimen in terms of toxicity and survival in these patients. III. Assess the acute neurotoxicity of continuous infusion cladribine in these patients. IV. Assess this regimen in terms of response, local control, and time to progression in these patients. V. Define a dose level of cladribine associated with potentiation of acute and/or delayed radiation toxicity (radiosensitizing dose level).